1. Field of Invention
The present invention relates generally to substrate support assemblies for supporting and stabilizing a substrate within a machine that performs an operation on the substrate, and more particularly to a substrate clamping assembly of a stencil printer particularly designed to clamp edges of the substrate during a print operation.
2. Discussion of Related Art
In a typical surface-mount circuit board manufacturing operation, a stencil printer is used to print solder paste onto a printed circuit board. A circuit board, broadly referred to as an electronic substrate, having a pattern of pads or some other conductive surface onto which solder paste will be deposited, is automatically fed into the stencil printer. One or more small holes or marks on the circuit board, called fiducials1, is used to align the circuit board with the stencil or screen of the stencil printer prior to the printing of solder paste onto the circuit board. Once a circuit board has been aligned with the stencil in the printer, the circuit board is raised to the stencil by a substrate support, e.g., a table having pins, and fixed with respect to the stencil. Solder paste is then dispensed onto the stencil, and a wiper blade or squeegee traverses the stencil to force the solder paste through apertures formed in the stencil and onto the board. As the squeegee is moved across the stencil, the solder paste tends to roll in front of the blade, which desirably causes mixing and shearing of the solder paste so as to attain a desired viscosity to facilitate filling of the apertures in the screen or stencil. The solder paste is typically dispensed onto the stencil from a standard cartridge. After the print operation, the board is then released, lowered away from the stencil, and transported to another station within the printed circuit board fabrication line.
There are a number of well-know methods for clamping the circuit board so that it is stabilized during the print operation. One such method is disclosed in U.S. Pat. No. 5,157,438 to Beale, which discloses a clamping mechanism having very thin foils that project over opposite edges of the circuit board for securing the circuit board to the support assembly. One disadvantage associated with this approach is that the foils are attached to their respective rails by adhesive. The arrangement is such that when a foil fails, the entire foil and rail must be replaced. It is expensive to replace the entire rail assembly, when only the foil is damaged.